1. Field of the Invention
The present invention relates to a control system for an automatic transmission for a vehicle, and more particularly, to a control system for an automatic transmission for a vehicle, wherein the shifting of the automatic transmission is prohibited when a slipping state of the vehicle it, determined.
2. Description of the Prior Art
When a vehicle provided with a vehicle speed detecting means for detecting a vehicle speed, on the basis of a rotational speed of a rotatable member of an automatic transmission, is started on ice having a small friction coefficient, the rotational speed of the rotatable member is increased if drive wheels slip. For this reason, the vehicle speed detecting means detects a higher vehicle speed, but the actual vehicle speed remains lower, because gripping between the drive wheels and the road surface has been lost. In this case, because the vehicle speed detecting means detects the higher vehicle speed, there is a possibility that the automatic transmission is upshifted, whereby the number of rotations of the drive wheels is further increased to exert an influence on the durability of a differential or the like.
A control system is known from Japanese Patent Publication No. 64-7259, which is designed, so that when the rotational acceleration of a rotatable member of the automatic transmission becomes equal to larger than a predetermined value, it is determined that the drive wheels are in slipping states, thereby prohibiting the upshifting.
When the vehicle provided with such a control system for the automatic transmission, travels at a high speed on a usual road surface, if a hard braking is carried out which locks the drive wheels, the automatic transmission is downshifted to a first gear shift stage. When the braking is released from this state, the number of rotations of the drive wheels restoring the gripping is steeply increased from zero. For this reason, there is a possibility that the control system misjudges that the slipping of the drive wheels has been generated at the starting of the vehicle on an ice or the like and as a result, the upshifting of the automatic transmission is prohibited to generate over revolution of the engine.